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LIBRARY 
STATE PLANT BOARD 

September 1953 E-864 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 



DUSTING STORED LOUISIANA SWEETPOTATOES 
TO PREVENT AN INCREASE IN SWEETPOTATO WEEVILS 

By C. H. Gaddis, Division of Stored Product Insect Investigations, 
Bureau of Entomology and Plant Quarantine, and E. A. Epps, Jr., 
Feed and Fertilizer Laboratory, Louisiana Department of Agricul- 
ture and Immigration—' 



In the four crop years between 1949 and 1953 an estimated 7,047,554 
bushels of sweetpotatoes in storage in southwestern Louisiana were 
dusted to prevent a build-up of sweetpotato weevils ( Cylas formicarius 
elegantulus (Sum.)) during the warm spring months. Prior to 1949 the 
State of Louisiana required the destruction or removal of all stored 
sweetpotatoes within the sweetpotato weevil regulated area by April 1. 
This requirement was necessary to prevent a heavy emigration of 
weevils from the storage kilns to the nearby fields, or their widespread 
dissemination during the transfer of sweetpotatoes from storage kilns 
to packing houses or dehydrators. 

The application of insecticide dusts to stored sweetpotatoes kills the 
adult weevils as they emerge before they have an opportunity to mate and 
lay eggs. Therefore, during the warm spring months all adults are 
killed as they emerge from the stored sweetpotatoes or invade the 
storage from undusted stocks. This procedure gradually depletes the 
population until a level is reached consisting only of individuals with 
delayed cycles which remain within the sweetpotatoes for many months. 

The research that furnished the basis for this treatment, carried 
on from 1948 to 1950 at Lafayette, La., is reported herein. The studies 
included the biological evaluation of different dusts, the determination 



1/ The writers acknowledge with thanks the cooperation of W. E. 
Anderson (since deceased), S. J. McCrory and Roy Meek of the Louisiana 
State Department of Agriculture; G. Z. Smith, and J. B. McDevitt of the 
Louisiana Agricultural Experiment Station; Herbert L. Morgan and 
A. H. Yeomans, of the Bureau of Entomology and Plant Quarantine; 
Girard & Trosclair and other shippers; and various sweetpotato weevil 
inspectors. 






■ dues -nt of a suitable 

du.s Ii a , observations on 

on of )d for Ita practical 

utilization w .seen 1950 and I 

Exploratory Tes 

•ness and long* >i residues of DD'I, BHC, and 

chlordane emulsions; DDT suspensions; and DD'I and m- hlor 

dusts against the sweetpotato weevil were determined in tests using 
flat 1-foot squares of sweetpotato- slats. Adult weevils were con- 

d under an ; i'etri dish at the same spot on the square- 

approximately 30-day intervals. Ai hours they were removed to 

clean Petri dishes and held in the laboratory at room temperature for 
observation. The results of these tests appear in table 1. It will be 
noted that the dusts were the onlv forn.ulations that were 100 percent 
effective for 365 days. 



le 1 .- -Mortality of adult swe» vils confined for 2 hours 

on insecticide residues of various ages in the laboratory at room 
temperature 



Insecticide 


liliters 
to each 
test square 


Percent morta 
to residues o: 


when exposed 
ited age 




2 




77 


106 


1 A\ 


:■ 4 


365 














days 






DDT: 
















0.75 % emulsi' 


15 






100 99 


98 




- 


5% suspension 






100 


100 100 


100 




- 


10% dust 


I 








100 


100 


100 


BHC, nulsioi 


i 






87 






- 


' 


13.5 














>0% d 


u 






' 


100 


100 


100 



* • 



>nal t- 

potat 

StO: g. 

os am: :ice 

of weevil '.potatoes 



-3- 



the treated crates became infested as compared with 100 percent of those 
in the untreated check crates. 

In other tests the crates and the sweetpotatoes were treated with 10 per- 
cent DDT or 50 percent methoxychlor dust, and held over winter under 
the same storage conditions. In the spring 6 percent of the DDT-dusted 
sweetpotatoes showed some evidence of attack and a few of them contained 
one or two weevil larvae. Six percent of the rnethoxychlor-dusted sweet- 
potatoes showed some evidence of attack, but no larvae were found. 

Cage tests were then run to determine the effectiveness of 50 percent 
methoxychlor dust in the presence of a very heavy weevil population. 
Each cage held three crates of sweetpotatoes. In one cage two crates 
were dusted; the third crate was untreated and contained heavily infested 
sweetpotatoes from which weevils were in the act of emerging. An other 
cage held two crates of untreated sweetpotatoes and a crate of infested 
ones. Both lots were exposed for 24 days. All the sweetpotatoes in the 
check cage became very heavily infested, having an average of seven 
egg punctures per square inch of surface. The dusted sweetpotatoes 
were uninfested, although approximately 13., 000 dead adult weevils 
were found in the cage. 

Storage tests of approximately 700 bushels of sweetpotatoes were 
conducted in four commercial kilns. The sweetpotatoes were treated 
with 25 or 50 percent methoxychlor or 10 percent DDT dust with an 
orchard duster. Prevention of reinfestation from emerging weevils was 
complete except where dust coverage was inadequate. 

Tests were also made to compare the reinfestation in dusted sweet- 
potatoes by weevils originating from within the dusted stock with that 
by weevils originating from untreated stock. Fifty and 25 percent 
methoxychlor and 10 and 5 percent DDT dusts were used. One-half of 
the dusted sweetpotatoes were exposed to crav/ling weevils; the other 
half contained a few heavily infested sweetpotatoes and were isolated 
from outside infestation. It was found that few of the sweetpotatoes 
were infested where the weevils emerged from dusted stock. The 
infestation was much greater where the sweetpotatoes were exposed 
to weevils from untreated stock. This difference was apparently due 
to the fact that weevils emerging from dusted sweetpotatoes were killed 
by the poison before they were ready to lay eggs. 



Large-Scale Tests 



A large-scale experiment was set up for a further study of the 
possibilities of using DDT or methoxychlor dusts. This experiment 
was designed to determine (1) the efficiency of normal washing practices 
in removing the residues, (2) the relative ability of the various dusts to 
prevent spread of weevils to uninfested sweetpotatoes, and (3) the distri- 
bution of dusts when applied with an orchard duster. 



- 4 



A 1, re divided into four equal 

in a sep;. , foot room. An 

was reser-. m each lot. One of the 

following formulations o: Din or met!. !or was applied to each lot: 

25 percent wettal '.ders dusts. Both dusts and 

w« powders were included to observe possible differences in 

remov The i rates of sv. atoes were dusted a layer 

is they were placed in the storage rooms, with an orchard- 
type power duster with a 4-inch outlet hose. One pound of dust was used 
for each 10 crates. Residues following washing were determined I 
analysis of samples withdrawn from four locations in each lot. The 
relative effectiveness of the various dusts was determined by confining 
adult weevils on dusted sweetpotatoes selected from each lot. The 
distribution of the dusts was determined by chemical analysis of the 
dust on sweetpotatoes from selected locations, and by biological tests 
with adult weevils confined on samples from the same locations. 

Residues After Washing 

One-bushel samples of sweetpotatoes were withdrawn from the front 
and back, top center.and bottom center of each lot after the 1st, 2nd, 3d, 
4th, 5th, 6th, 11th, 15th, and 20th weeks. Each sample was divided into 
two 5-pound subsamples, one washed and the other unwashed. The un- 
washed subsample was selected at random after the crate had been 
dumped out on the apron of a standard sweetpotato washer at a com- 
mercial packing shed. The remainder of the bushel was then washed, 
and a 5-pound subsample was selected at random and paired with the 
shed subsample. The samples were immediately transported I 
Baton R • , I. .. sidues were analyzed at the Feed and 

I; this analysis insecticide residues v. ived I pping 

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The washing procedure reduced the toxic residues tc a very low level. 
The average residue from each formulation, found after washing, was as 
follows: 

P. p. m. 

DDT, 25% 5.0 

10% 3.8 

Methoxychlor, 25% 3.8 

10% 2.1 

The efficiency of washing did not seem to be related to the age of the 
residue. The residue was fairly constant regardless of the time elapse. 

Relative Effectiveness of Four Dust Formulations 

Two series of tests were run to determine the effectiveness ci the 
four DDT and methoxychlor dusts in controlling weevils. 

In the first series the four formulations were compared. Thoroughly 
dusted sweetpotatoes were placed in cages, and 1.00 adult weevils were 
introduced into each cage. At the end of 10 days the sweetpotatoes were 
examined for punctures and weevil-mortality records were taken. The 
tests were repeated three times at about 10-day intervals with the same 
sweetpotatoes but a fresh lot of weevils each time. At the end of the 
30-day period sweetpotatoes were examined for punctures, eggs, and 
larvae, and the residue was determined. The results of these tests, 
given in table 3, show that all the dusts except the 10 percent methoxy- 
chlor were capable of killing all female weevils before they were able 
to deposit eggs in the sweetpotatoes. 

Table 3. --Effectiveness of DDT and methoxychlor dusts against three lots of adult 
sweetpotato weevils confined consecutively for about 10 days on 15 completely 
dust-covered sweetpotatoes and residues found at end of the tests 



Dust 


First let 


Second lot 


Third lot 


Number 
of punc- 
■ tured 


Number 
of 


Number 
of 


Residue, 
















p. p.m. 




Dead 


Alive 


Dead 


Alive 


Dead 


Alive 


sweet- 
potatoes 


eggs 


larvae 




DDT 
















25% 


229 


219 


210 


1 








145.0 


10% 


216 


215 


224 


6 








94.6 


Methoxychlor 
















25% 


206 


206 


223 


15 








165.7 


10% 


189 5 


217 1 


221 


Many 


Many 


7 


111.7 



-8- 



In the s* series • thoroughness of dust coverage at two 

'orage room ttom levels in the sweet 

to crates we Justs were 1 and the samples 

: and infested in the same manner as in the previou s. 

From the results in table 4 it :ent that the dust coverage 

was very uneven. 



Table 4. --Range of dust deposition in the storage room and crate- 
shown by weevil mortality and residue analyses 



Dust 



Location 
in crate 



Location 
in room 



•iber of wee 



D< : 






sidue at 
end of test, 
p. p. :: . 



DDT, 25 'o Bottom 


Back 


313 


: 


13.9 




Front 


317 







Top 


Bac k 


317 





108.5 




Front 


322 





301.1 


10% Bottom 


Back 


255 


70 


. 




>nt 


286 


1 


. 


Top 


Back 


305 





• . 




>nt 


316 





.. 


Methoxychlor, 25 o Bottom 


Bac k 


• 




19.9 




>nt 


298 


14 


• 


Top 


Ba 


291 


23 


61.2 




>nt 


314 


1 


10 . 


! • • om 


Bac k 


98 


- 


. 




Hit 


22 








ck 
















1 1 



I 1 (unt: 









-9 - 



Distribution of Dust in Storage Rooms 

The distribution of dusts applied with a power duster to one 
layer of crates at a time was quite uneven. The deposits were much 
greater near the doorway than at the rear of the rooms, and most of the 
dust in each crate was on the top layer of sweetpotatoes. The chemical 
analyses reported in table 4 demonstrated that the dust deposits on the 
sweetpotatoes from the bottom layers in the crates ranged from about 
14 to 47 percent of the deposits on the top layer of sweetpotatoes in the 
same crates. The differences in mortality of weevils confined to these 
dust deposits are particularly marked in the 10 percent DDT and methoxy- 
chlor samples. These tests emphasize that with such unequal distribution 
of dust a much heavier application of insecticide is needed to effect high 
mortality. 

Development of a Duster for Crated Sweetpotatoes 

It was evident from the storage-room tests that better distribution of 
dust within the crates was needed. Owing to the great amount of sweet- 
potatoes that would be involved if a dust treatment proved feasible, it 
appeared impracticable to remove them from the crates in order to obtain 
better coverage. Such a practice would also cause some bruising or 
other injury. Therefore, attempts were made to develop a duster that 
would be better adapted to this special purpose. 

Preliminary tests showed that, if a small amount of dust could be 
puffed, or "exploded," into a crate of sweetpotatoes, the distribution and 
coverage would be fairly satisfactory. A duster was designed to accom- 
plish this objective. It dusts one crate at a time as a line of crates is 
passed beneath the nozzle on a roller conveyor. About 24 grams of 
dust (1 pound per 20 crates) is discharged by means of compressed air 
into each crate when it is directly beneath the nozzle. 

The duster (figs. 1 and 2) consists of an upright hopper that will 
hold about 20 pounds of dust, a revolving disk carrying four cups each 
of which holds the exact dosage for one crate, an arrangement to dis- 
charge compressed air into the cup in dusting position, a hood that 
directs the dust into the crate, a tunnel to shield the crates while being 
dusted, and a length of roller conveyor. The dust hopper has a very 
gradual taper. An agitator has been provided to prevent the dust from 
clogging but is seldom needed. The revolving disk is designed to rotate 
in such manner that the cups pass beneath the hopper, where they are 
filled with dust. The measuring cups, or "discharge cartridges," have 
two half-round gates attached to the bottom by hinges. Each gate has an 
arm to which lead counterweights are attached. The gates are so con- 
structed that they normally remain closed, even when the cups are filled 
with dust. Appropriate handles and stops have been provided whereby 






revolving disk can ";lling position to the point 

where the dust is • be The filled cups are visible during 

so th. e checked. 

The compressed air us' ropel the dust into the crates is released 

pressing a trigger momentarily on . e. This air pressure opens 

the two gates in opposite dire< , nd the dust is "exploded" and eve- 

spread into the sweetpotato crate. A pressure of 100 to 125 pounds per 
square inch gives best results. A somewhat funnel-shaped hood has been 
provided to confine and direct the dust from the point at which it is re- 
leased into a tunnel-like enclosure through which the sweetpotato crates 
pass. The tunnel is approximately 8 feet long, and is large enough to 

mit a section of roller conveyor and sweetpotato crates to move 
through freely. 

Dust is released in the tunnel at a position directly above the third 
crate from the entrance. It was demonstrated that crates can be passed 
through the duster as fast as they can be fed onto the conveyor, or about 
600 to 800 an hour (fig. 3). As many as 8,000 per day have been dusted 
in commercial operation. Uniform coverage has been obtained with a 
dosage of 4/5 ounce of 10 percent DDT per crate. 

This duster has been adopted for practical use, and more than 50 
machines have been manufactured. 

1 •• tment to Remove !)I) I :"•■■ □ ■■:• Wasl er 

Waste water from sweetpotato-washinp nes is normally dis- 

posed of through an open drain to ditch, s. H 

only •• million of DDT is b to fish ; 

possibly to Livestock and wildlife. A >f the waste water fr 

DD I itpota.1 ild be disposal 

it treatment to DD 1 . L -th bluegill 

and bream showed that the DDT in tl -ould be rev 

filtration. 

I • ition ti n. 

A , 

I, in addil Lon to D ta of 

i 

■ i 

■ 
ii 

■ 

DDT. 

■ 



11 - 



would give adequate removal of DDT. After passage through a settling 
pit the waste water treated with lime was found to be free of DDT at a 
point 100 yards from the washer. 

Application of Dusting Procedure for Quarantine 

The results of the research described above were reviewed by officials 
of the Louisiana Department of Agriculture and Immigration and the Bureau 
of Entomology and Plant Quarantine. The 10 percent DDT dust was 
selected for commercial use because it was generally available, it was 
the cheapest of the formulations that gave satisfactory performance, and 
the residues were removed by the usual washing practice to a level con- 
sidered to present no hazard to the consumer. 

This consideration was later confirmed by analyses of samples of 
sweetpotatoes taken from 12 commercial lots after they had been washed 
in preparation for marketing. The DDT residues after washing ranged 
from 0.1 to 3.6 p. p.m. Sweetpotatoes from the same samples analyzed 
after boiling or baking showed no DDT in the cooked sweetpotato pulp. 
The Schechter-Haller colorimetric method for DDT determination was 
used in these analyses. 

On October 3, 1949, the State of Louisiana revised its quarantine 
regulations as follows: 

The deadline for the shipment of sweetpotatoes from the quarantine 
area of Louisiana will be extended from April 1 to May 15, provided: 

1. The sweetpotatoes to be marketed after April 1 are dusted with 
10 percent DDT dust before March 1 of each year. 

2. The applicator used in applying the DDT dust is approved by 
the Department. 

3. The dust must be applied under the supervision of a represent- 
ative of the Department or the Bureau of Entomology and Plant 
Quarantine. 

4. The wash water from the sweetpotatoes that have been dusted 
with DDT must be disposed of in a manner approved by the 
Department. 

All sweetpotatoes not dusted with DDT by March 1 will have to be 
moved out of the quarantined area, as in previous years, by April 1. 

These regulations were further revised on January 26, 1950, to re- 
move the May 15 deadline and extend the shipping period indefinitely for 
all sweetpotatoes dusted with 10 percent DDT before March 1. 






d 

Dus* toes with 10 percent DD *.he 

n has been in pra< >ur 

• Is were dus 1 *he 1949-50 crop. 

s from tl 
, 00 bushels from the 1952-53 
ve been made of the dusted sweetpotatoes. [1 *as 
_;ing from them r; : d long enough to r 

st swi itoes. 

I sweetpotatoes that had been dusted were I ed to 

r ie length of time that weevils m: :n in them before 

em- It was found that a few adults were still present after 5 months' 

storage at prevailing and that some remained swet 

es were held at 70° F. storage for 4 months. 
At first the di st treatment was used primarily to prolong the storage 
and shipping seasons. Shippers have observed the benefits derived from 
dusting, and during the last two seasons many of them dusted sweet- 
potatoes that were to be kept in storage only for short periods. In the 
process of culling and grading dusted and undusted potatoes it was found 
weevil damage was much higher in the undusted stock. The treat- 
• proved so effective during the first three seasons that practice, 
all sweetpotatoes placed in storage in the weevil-infested area in the fall 
of 1952 were dust< 

Summ. 

Studie.- 1948 to 1952 to explore the poss of 

dusting table-stock sweet I - es ill storage to pr< t< ige 

formicari s • .• :ntulus (Sum.)). 

I. sts sh hat n lues of 1 DDT or 50 pe: 

s up to 365 
. 

•• dust form »nd 10 

• DD I - W0 

i 

first threi 

toes. 1 
in kill: 

I] ) 

. . ■■ . 
t DDT, l.fl . . DD 

■ ■ 



13 - 



The average residues were in the same order of amount as their toxicity 
to weevil adults. The residues after washing did not increase with their 
age on the sweetpotatoes. 

As the distribution of dust when applied with the power duster was found 
unsatisfactory, a duster was developed in which compressed air was 
used for dust dispersion. This duster was found to give good coverage 
throughout the crate at a dosage of 4/5 ounce of 10 percent DDT per crate. 
The crates were treated one at a time at the rate of 600-800 crates per 
hour. This duster has been adopted for commercial use, over 50 
machines now being in use. 

DDT contamination of waste wash water was prevented by adding lime 
to the waste, which caused flocculation and settling of the DDT and mud 
present in the water. 

On the basis of these tests, the Louisiana State Department of Agri- 
culture revised its quarantine to extend the possible storage time from 
April 1 to May 15, provided that the sweetpotatoes were dusted with 
10 percent DDT before March 1. In a later change the storage time was 
extended indefinitely. This treatment has had a very beneficial effect 
economically. Over 7,000,000 bushels have been treated in the last four 
crop years, and during the last two years many shippers have been 
dusting all stored sweetpotatoes even if stored only for a short time. 

Literature Cited 

(1) Wichmann, A. J., Patterson, W. I., Clifford, P. A., Klein, A. K., 
and Claborn, H. V. 

1946. The determination of DDT as spray residues on fresh 

fruit. Three independent methods. Assoc. Off. Agr. 
Chem. Jour. 29: 188-218. 



3 1262 09239 6497 




I- . re 1.- -Dus r appl 

with ssed air to crates of 

sweetf otatoes. A, H 

B, ressed-air ti 

C, -shaj • 
of the tunnel. 






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.de of hopper with 
4 disk to • ire attached 

the four measuring cups with cou:. 
: outlets. 



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